I used to believe the same way, but Stevebfl pointed out in an earlier post that they do "wear out" over time. I don't know why or how, but they do slowly become less effective.
I wish I had a better memory or had saved the post, but if you search hard enough under his user name, we might be able to find it.
Arthur Dalton also told me he hooked up different O2 sensors under a scope and watched them "working" and noted how different brands work faster or slower than other brands.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".